Wearables

Low-power wearables may soon bid adieu to batteries and start drawing energy generated by body heat and movement, and ambient energy from the environment.

Consumer electronics devices are getting smaller but conventional batteries are not, and it's important to start implementing new energy harvesting techniques to keep devices powered for long periods of time, researchers said at the Hot Chips conference in Cupertino, California, on Sunday.

Energy harvested from body heat, motion and ambient light could be used in medical implants, monitoring sensors and disposable medical patches, said Yogesh Ramadass, lead design engineer at Texas Instruments, during a presentation at the Hot Chips conference.

The technologies are still emerging, but the chip performance and energy efficiency of some wearables are reaching a point where it has started becoming "convenient for us to replace the battery and replace it with ambient energy," Ramadass said.

Energy harvested from the body and environment is in the microwatt range, so it can't be used for smartwatches or fitness trackers, which draw milliwatts of energy, Ramadass said.

"You shouldn't be thinking about a regular wearable devices like FitBit, smartwatch or others," Ramadass said in an interview on the sidelines of the show.

Smartwatches have displays and software that can drain batteries, while energy harvesters are better for wearables that collect and transmit bits of data at specific intervals.

Self-powered devices could make an impact in the context of the Internet of Things, said Massimo Alioto, associate professor at the National University of Singapore.

There will be billions of Internet-connected devices supplying real-time information in the coming year. Data-gathering instruments today are designed around the size of batteries, and self-powered devices could resolve some power and size issues, Alioto said.

The researchers said that energy harvesting technologies could be relevant in smoke detectors, alarm sensors, smart meters and even remote controls.

There will be 26 billion Internet-connected devices by 2020, according to Gartner. Sensors will be used in wearables, industrial equipment, energy monitors, telematics systems, home appliances and other "intelligent" appliances, Gartner said. Another research firm, PricewaterhouseCoopers, is predicting IOT to become a multitrillion dollar industry by 2020.

The energy efficiency of circuitry plays a major role in determining the size, weight and operating time of self-powered IOT devices, TI's Ramadass said.

"Typically in indoor or wearable situations we're talking about few tens of microwatts, in an industrial situation or sunlight we're talking about few milliwatts per centimeter square or cube... depending on what the harvester is," Ramadass said.

One energy harvesting technology is solar energy, which is already used in calculators and other devices, but could also be used in data-gathering instruments that transmit information wirelessly. The average power generated through indoor lighting would be few tens of microwatts, while sunlight would generate milliwatts of power. Research is ongoing to improve the efficiency of energy generation, Ramadass said.

Wearable devices like medical sensors could also generate energy from thermoelectric cells, which rely on ambient heat. Thermoelectric cells can take the body heat and apply it to electrons, after which it runs through the process of generating energy. The cells can generate 30 to 40 microwatts of power, roughly the same as solar cells, and when placed in a series, can generate more energy, Ramadass said.

"You can think of a wearable kind of system where you are applying a thermoelectric device and the temperature difference between the body and ambient [heat] is going to provide the energy to power a system," Ramadass said.

A lesser practical source for wearables would be piezoelectric, which relies on motion and vibration to generate electricity. Walking or running could power wearables, but its not practical to place cells in shoes, Ramadass said. This technique is more practical in industries like oil and gas, where continuous motion in a pipeline could power up sensors.

Energy could also be harvested through ambient mechanical vibrations such as electromagnetic waves. These are not good for wearables, but more for stable installations in industries or homes where few tens of microwatts of power can be generated on a consistent basis.

Wireless charging is also available via inductive coupling, but it's not "truly wireless," Ramadass said. A medical patch with a transmitter coil on a patch or t-shirt could power up and collect data from a medical implant inside a body. That is similar to RF wireless signals, in which a reader can power up and receive data from an RFID tag.